The present invention relates to an audio device which corrects a delocalization of a center sound image or an asymmetrical expansion of a sound field in a reproduction sound field, such as a car cabin or a listening room, to thereby provide a natural sound space to listeners.
In a conventional audio device, right- and left-channel speakers 5 and 6 are disposed in a reproduction sound field space 4, such as a listening room, as typically shown in FIG. 17A. When a listener hears a stereophonic sound or the like at the center in front of the speakers 5 and 6, a center sound image C, such as vocal, is localized in front of the listener. When the listener listens at a position asymmetrically located with respect to the speakers 5 and 6, the center sound image C is delocalized, thereby failing to produce a natural sound field space.
A car-carried audio device is known as a typical case where the center sound image C is likely to be delocalized. The car-carried audio device is used in a special place, viz., within a car cabin of an automobile. Accordingly, it is common practice that the left- and right-channel speakers 1 and 2, as typically shown in FIG. 17B, are disposed at a position located asymmetrical with respect to a passenger (listener) Therefore, the center sound image C, such as vocal, to be localized in front of the listener is delocalized to a position closer to the speaker 2 disposed closer to the listener.
To cope with the delocalization problem of the center sound image within the car cabin, there is proposed car-carried audio devices with a balance adjustment function and a time alignment function.
In the car-carried audio device with the balance adjustment function, as shown in FIG. 17C, an output level of the speaker 2 located closer to the listener is reduced to be lower than the output level of the speaker 1 located farther from the listener by an amplitude adjustment circuit 7. As a result, the sound pressure levels of the right- and left-channels are balanced with respect to the listener to localize the center sound image C in front of the listener.
In the car-carried audio device with the time alignment function, as shown in FIG. 17D, an audio signal is supplied to the speaker 1 located farther from the listener, and after some time elapses, an audio signal is supplied to the speaker 2 closer to the listener, whereby the right- and left-channel sounds reach the listener at the same time, and the center sound image C is localized in front of the listener.
A head related transfer function (HRTF) correction method is known. In the HRTF basis correction method, a sound field of a concert hall or the like is simulated or a sound image is localized in a desired direction by controlling a transfer function (amplitude and phase characteristics) of a space between a speaker and the ears of a listener. Attempt has been made to correct a delocalization of a sound image or to enlarge a sound field by applying the HRTF correction method to the car-carried audio device.
The audio devices with the balance adjusting function and the time alignment function are capable of localizing the center sound image in front of the listener, indeed. However, it is difficult to remove the asymmetric expansion of a sound field as viewed in the horizontal direction.
In the case of using the head related transfer functions, a great amount of audio signals must be digital processed for an extremely short time. Therefore, the signal processing circuit of a large scale and high speed is required.
FIR (Finite Impulse Response) digital filters, for example, are used for the signal processing circuits to realize the head related transfer functions. In this case, a great number of filter coefficients and delay elements are required so as to satisfactorily correct complicated sound field characteristics. Increase of the circuit scale and processing speed is unavoidably imparted on the signal processing circuit.
Even if the deformation of the sound field is corrected by the HRTF basis correction method which uses the signal processing circuit of large scale and high speeds, the correction is effective only under limited conditions. If the listener is constantly static, the transfer functions in a space ranging from the right and left speakers to the right and left ears of the listener including his head remain unchanged. Therefore, the correction improvement is achieved under that condition. Actually, in the car-carried audio device, the listener frequently moves his head in the driving operation, and in the audio device installed in a living room, the listener is not always static. Accordingly, the transfer functions in a space from the right and left speakers to the listener vary, and it is impossible to quickly change the head related transfer functions following the distance change.